In the semiconductor industry, the patterns of conductors on circuit boards and the transistors on microchips are printed or etched onto substrates or wafers using a photoresist. For instance, many millions of micron-sized devices can be fabricated simultaneously and reliably on silicon substrates via the application of a layer or multiple layers of photoresist. A photoresist is a polymeric coating that can change properties upon exposure to light and that can resist etching, ion implantation, metal deposition, etc., to protect the substrate beneath where required. For example, in a situation where a chemical treatment etches away some of the substrate, the photoresist can protect the other regions of the substrate, after which an appropriate reagent such as plasma is utilized to remove or strip the remaining photoresist.
However, as the device feature size of integrated circuits continues to be scaled down, removal or stripping of the photoresist becomes more difficult, particularly in high aspect ratio (HAR) applications where the ratio of the length of an object to its width is greater than about 25, such as greater than about 50 (i.e., when the ratio of the height of a cavity to the width of a cavity is greater than about 25, such as greater than about 50), or in applications where the critical dimension (CD) of the substrate becomes smaller. Further, photoresist removal with zero substrate loss while simultaneously maintaining gate material integrity is critical with high dose implant strip (HDIS) and descum processes.
One solution as it pertains to conventional oxygen-based plasma stripping is to increase power into the plasma. However, this results in oxidation damage, for example, to the metallic liner coating the inside surfaces of HAR holes. Such oxidation, in turn, results in increased sheet resistance and is prevalent when oxygen-rich reducing chemistry is used in conjunction with high-k metal gates (HKMGs), thus negating the electrical advantages seen with the use of HKMGs. Further, removal of the oxidized layer results in severe CD change and loss of contact liner thickness, which is already thin.
Meanwhile, the use of pure reducing chemistry is well established when stripping photoresist in the presence of metallic surfaces, but it has intrinsically low photoresist removal rates, even at high power, and typically, purely reducing plasmas have an order of magnitude lower strip rate than oxygen-based plasmas of equal power and gas feed density. Moreover, residual photoresist is often left at the bottom of the HAR holes, even when the process time is extended many times beyond that required to strip a monolithic photoresist film of equal thickness. The different behavior is presumed to be due to the loss of reactive species to surface reactions along the container walls of the HAR holes, thus preventing sufficient reactant density at the surface of the holes.
Thus, a need exists for a method of removing photoresist in an economically feasible manner that results in minimal to zero substrate loss and does not result in the oxidation of the metallic lining in such holes. A need also exists for a process for oxidized layer removal from conductive materials that does not result in thickness loss.